In the design of a nail for nailing into a wooden work piece, the principal critical factor is the so-called "holding power" of the nail in the work piece. Obviously, in the majority of nailing applications, it is desirable to provide a nail with the maximum possible holding power, i.e., resistance to pullout from the work piece. In the past, the majority of nails have been simple cylindrical wire nails having a predetermined diameter. Generally speaking, it has been understood that the greater the diameter of the nail, the greater is its holding power. However, the greater the diameter of the nail, the greater is the amount of metal incorporated in the nail, and thus the cost of manufacturing the nails is increased. Many proposals have been made in the past for attempting to provide a nail with increased holding power, for a given cross-sectional diameter of wire, as compared with a conventional cylindrical wire nail. These proposals have involved the use of various forms of knurlings or serrations, or other formations along the nail shank. These proposals are based upon the theory that the knurlings or serrations on the nail shank will somehow provide a more secure engagement with the fibres of the wooden workpiece. In practice, however, it has been found that they have produced very little, if any, increase in holding power. Another proposal has been the provision of a nail with a generally square cross-sectional shape, which is twisted longitudinally along its axis. These nails are known as "twisted wire" nails. These nails do in fact have an increased holding power. However, such nails present certain difficulties. In the first place, they are not always suitable for insertion by nailing applicator machines, since a nail of twisted wire preferably should rotate as it goes into the workpiece. Nailing applicators are not always capable of permitting the nail to rotate. In addition, the twisted wire nail presents an uneven configuration along its shank, and consequently it is not always easy to form such twisted wire nails into a nailing stick suitable for use in a nailing applicator machine.
Extensive research has been carried out by various research facilities, and by various companies into factors affecting the holding power of nails. As a result of this research, it has been substantially established that the principal factor affecting the holding power of a nail is simply the degree to which the wooden fibres are displaced transversely to i.e. "across", the axis of the grain of the wood. In other words, the general rule is that the greater the dimension of the nail, transverse to the axis of the grain, the greater will the displacement of the wood fibres, and the greater will be its holding power. Conversely, features of the nail parallel to the axis of the grain have little, if any, effect on its holding power.
There are also other factors which must be taken into consideration, in the particular case of nails designed for formation into nailing sticks for use in nailing applicators. Generally speaking, it is desirable that a nailing stick shall incorporate as many nails as possible in a given length of stick. This means that the nailing applicator can be used for a greater length of time before it is necessary to insert a fresh stick of nails. Obviously, if the nails are spaced apart a substantial distance between one another in the stick, then in a given length of stick, there will be a smaller number of nails. Thus, the nailing stick must be replaced more often. Thus, the downtime for the nailing applicator itself is increased, and the individual worker's output is reduced.
This factor is well understood, and in order to limit the spacing between the nails it has been customary to provide nails for nailing sticks, with "clipped" heads. In nails of this type, the nail head consists of a an incomplete circular formation, with an edge extending around an arc, and a linear side edge between the ends of the arc, which is essentially co-planar with the surface of the nail shank. In this way, the nail heads may be overlapped with one another, in the forming of the stick, and the shanks of the nails may be laid in contact with one another. This will produce a nail stick in which each nail is axially offset from the next adjacent nail by about the thickness of the head. The stick will thus have an essentially diagonal axis. Nailing applicators are designed to receive nailing sticks of this type, and such nailing sticks using "clipped" headed nails are common in the industry. One of the problems however associated with the use of such clipped heads is that they somewhat reduce the holding power of the nail. If such clipped heads are inserted, with the linear side edge of the head parallel with the longitudinal axis of the grain (i.e. along the grain), then there is a tendency for the holding power to be reduced, or, conversely, the nailing applicator will tend to force the nail too far into the workpiece. Either result is undesirable.
In one earlier proposal, nails having shanks of square cross-sectional shape, but without having an axial twist, have been used. These nails were formed with generally clipped heads, so that they could easily be assembled into a nailing stick with the sides of the nails abutting against one another and the heads overlapping. One of the disadvantages of this type of nail is that the square shape of the cross-section caused in effect a reduction in the holding power of the nail. The lateral dimension between the two opposite sides of the nail was less than the diameter of a cylindrical cross-section nail of an equivalent weight of wire. Consequently, there was a somewhat lesser degree of displacement of the wood fibres transversely of the axis of the grain, and a lower holding power resulted. In order to attempt to overcome this problem, teeth or notches were struck out from opposite sides of the nail, with the hope of providing a better hold on the wood fibres. In practice, however, this was found to be no more effective than the use of a cylindrically shaped cross-section.
In fact, the teeth or notches could actually reduce the holding power. Thus if the teeth or notches were formed with sharp or pointed ends, they would tear the wood fibre. Nevertheless some manufacturers still insist on achieving the sharpest possible teeth or notches.
Clearly, it is desirable to provide a nail particularly suitable for use in the formation of nailing sticks for use in nailing application, having a holding power which is increased as compared with a cylindrical cross-section nail of an equivalent weight of wire, and yet having a cross-sectional shape which may be easily formed into a nailing stick.